1. Field of the Invention
The present invention relates to an electron gun which can attain excellent focusing characteristics balanced in the whole area of a fluorescent screen and a favorable resolution owing to the small diameters of electron beam spots on the fluorescent screen and a cathode-ray tube containing the electron gun.
2. Description of the Prior Art
In cathode-ray tubes each having an electron gun configured of a plurality of electrodes, a deflection device, and a fluorescent screen, various techniques have heretofore been proposed as expedients for obtaining a good picture over the surface of the fluorescent screen from the central part to the peripheral part.
By way of example, as disclosed in the official gazette of Japanese Patent Application Laid-open No. 103752/1983, electrodes constituting a main lens are formed with electron-beam passing holes which are noncircular.
With the electron gun of this structure, astigmatism corrections are made by mounting an electric-field correcting plate only within an anode, because appropriate astigmatism corrections are incompatible with the appropriate static concentration of an electron beam on the fluorescent screen.
In the above application, there are two reasons why the astigmatism corrections are required. The first reason is that, since a color cathode-ray tube now in use, having three electron guns arrayed in line employs a non-homogeneous deflecting magnetic field in order to simplify a convergence circuit, resolution at the peripheral edge of the screen of the cathode-ray tube is prevented from being lowered by the action of the deflecting magnetic field.
The second reason is to correct an astigmatism which arises when the electron beam passes through the noncircular electron-beam passing holes as stated in the official gazette.
Especially in the case where the electron-beam passing holes are formed in the noncircular shape as stated in the official gazette, the astigmatism lowers the resolution attributed to the deflecting magnetic field.
Accordingly, the amount of the astigmatism corrections based on the electric-field correcting plate is large and actually the astigmatism is not negligible even when compared with the spherical aberration of the main lens.
Therefore, the diameter of an electron beam spot on the fluorescent screen cannot be reduced to an extent corresponding to the enlargement of the electron lens attributed to the noncircular electron-beam passing holes.
In an electron gun disclosed in the official gazette of Japanese Patent Application Publication No. 36225/1989, the electron-beam passing holes of electrodes constituting a main lens are made circular, and an astigmatism is corrected only on the anode side of an electrode pair consisting of the focusing electrode and the anode which constitute the main lens.
The official gazette of Japanese Patent Application Publication No. 1344/1990 discloses improvements in the aforementioned structure of Japanese Patent Application Publication No. 36225/1989, wherein an astigmatism correcting portion mounted within the anode is shaped into a slot which extends in the in-line array direction of three electron guns less than the diameter of the electron-beam passing hole provided in the anode which is constructed of a metal plate.
In addition, the official gazette of Japanese Utility Model Registration Application Laid-open No. 18164/1975 discloses an electron gun wherein "U-shaped" auxiliary electrodes for electric field corrections are mounted inside respective electrodes which constitute a main lens.
The official gazette of Japanese Patent Application Publication No. 7375/1985 discloses an electron gun wherein the part of an anode on the side closest to the fluorescent screen is provided with an electric-field correcting portion cut and mounted in parallel with the direction of an in-line array while a focusing electrode is provided with a cut-and-mounted part between the electron-beam passing holes of the anode and the focusing electrode in a direction perpendicular to the in-line array direction.